1. Field of the Invention
The present invention relates to regulating the voltage of a second output of a multiple output forward converter by switchably connecting the second output to a first closed-loop regulated output using a synchronized switch and tapped coupled inductor.
2. Description of the Related Art
Cost and efficiency are almost always major focal points for computer designers, especially designers of personal computer systems. The power supply is receiving an increasing amount of attention where designers are constantly attempting to improve efficiency. Significant advances have been made so that at least the same amount of power or more is being delivered by smaller units. Nonetheless, further improvement is essential to keep pace with the demands of the computer market.
A typical computer system includes many components having different power and voltage requirements, where the components generally include logic circuits, video displays, hard disk and floppy drives as well as keyboards, mouse units, cooling fans and various other components. A switching power supply is preferred for use with a computer system since it provides one of the better known ways to convert a raw AC or DC voltage to several DC voltage levels required for the various components in the computer system. Depending upon the application, DC-to-DC or AC-to-DC switching converters are typically used. Switching DC-to-DC converters convert an unregulated DC voltage, typically from a battery, to the various DC voltage levels required at the outputs. More commonly, a switching AC-to-DC converter is used which first converts AC power to a large unregulated DC voltage using a filter and rectifier circuit, and which then converts the unregulated DC voltage to the various required regulated DC voltage levels through a converter transformer. The current through the primary of the converter transformer is usually controlled by a pulse width modulator (PWM) circuit. Although several types of converters are known, such as forward and flyback converters, the present invention concerns forward converters.
Some of the outputs of a power supply for a computer are required to be regulated at relatively tight tolerances, while other outputs require less regulation and may vary significantly more. For example, a 5 volt output usually provides power for the logic circuitry, which circuitry generates logic signals having information content, so that very little noise content is tolerated. The 5 volt supply, therefore, is usually designed with feedback circuitry implementing sophisticated regulation techniques to maintain the voltage level at 5 volts within specified tight tolerance levels under various load conditions.
Several outputs of the power supply are used to provide raw power to devices such as fans and disk drive motors, which can tolerate greater voltage variances so that the output requires less regulation. For example, a 12 volt output is typically provided to power electric motors used in disk and floppy drives as well as cooling fans. Less sophisticated regulation techniques are used for the 12 volt supply and usually involves open-loop control. Although the 5 volt supply is generally connected to a roughly constant load, the load applied to the 12 volt supply is relatively erratic, and varies from no load at all to sudden large loads drawing large surge currents, such as when electric motors are initially activated. Specifications usually allow wider tolerances during surge conditions. It is noted that regulation of the 12 volt supply is less problematic under average or even surge load conditions. However, regulation becomes more difficult when little or no load is applied since the voltage tends to float above the highest voltage level allowed by the specifications, which is undesirable because it could damage some of the electronic devices connected to the output. Since low-load and even no-load conditions of the 12 volt supply are common, some form of regulation circuit is necessary to keep the voltage level within specification. There have been several regulation circuits which have been used. Most of these regulators exhibit undesirable characteristics, however, such as inefficiency and excessive cost due to relatively expensive regulation components or relatively high parts count and complexity.
FIGS. 1A-1D illustrate some prior art methods which have been used to regulate an open loop output of a forward converter switching power supply. The simplest method is to provide an output preload or bleed resistor 34 across the output so that a load is always provided. An output preload regulator 20 is shown in FIG. 1A, which is a forward converter generally including a primary circuit 21 coupled to a primary coil 22 of a converter transformer T, where the transformer T also includes a secondary coil 24. The dotted terminal of the secondary coil 24 is coupled to the anode of a forward diode 26 and the undotted terminal of the secondary coil 24 provides the 12 volt return signal, referred to as 12 V RTN. A diode 28 has its anode connected to the 12 V RTN signal and its cathode connected to the cathode of the diode 26. A coupled storage inductor 30 has one terminal connected to the cathodes of the diodes 26 and 28 and its other terminal providing the 12 volt output signal, referred to as 12 V. A load capacitor 32 is coupled between the 12 V and the 12 V RTN signals.
It is noted that the output preload regulator 20 only shows one output leg or portion of a multiple output switching power supply, where each output provides a different voltage level. Each output is preferably implemented as a forward converter and includes an output series storage inductor, such as the coupled storage inductor 30, which is used to store energy. All of the storage inductors of all outputs are typically wound around a single magnetic core in forward converters, which is the reason the storage inductor 30 is referred to as being "coupled."
In general, the primary circuit 21 is preferably a pulse-width modulated (PWM) switching circuit, which causes current to flow into the dotted terminal of the primary coil 22 during the forward portion of each cycle. The primary current causes current to flow from the dotted terminal of the secondary coil 24 through the forward diode 26 and through the storage inductor 30 charging the capacitor 32 and providing power to a load (not shown) coupled between the 12 V and 12 V RTN signals. Energy is also stored in the storage inductor 30. The primary circuit 21 then switches off for the remaining period of time of each cycle, referred to as the flyback portion of the cycle, which stops current flow through the primary and secondary coils 22 and 24 of the transformer T. This forward biases the diode 28 and current flows through the inductor 30, the capacitor 32 and the diode 28.
The transformer T, the values of the storage inductor 30 and the capacitor 32 are chosen to provide approximately 12 volts, where the inductor 30 and the capacitor 32 also serve to filter the output voltage of the 12 volt supply. If the load resistor 34 were not provided and no external load were present, the capacitor 32 would continue to charge to levels above 12 volts and above specified levels, which is undesirable. The load resistor 34 prevents the capacitor 32 from overcharging and roughly maintains regulation at the desired 12 volt level under various load conditions. A significant disadvantage of the output preload regulator 20 of FIG. 1A, however, is that the load resistor 34 wastes valuable power by converting a significant amount of power into heat. This creates a relatively inefficient design and also raises the internal temperature of the power supply.
FIG. 1B illustrates a magnetic amplifier regulator 36 which is another prior art method. Similar components as those in FIG. 1A are indicated using identical reference designators. The primary circuit 21 and converter transformer T1 are not shown for purposes of simplicity, yet the magnetic amplifier regulator 36 is also a forward converter which operates in a very similar manner as the output preload regulator 20. A magnetic amplifier inductor 38 is added which has one terminal coupled to the dotted terminal of the secondary coil 24 and has its other terminal coupled to the anode of the diode 26. An error amplifier and drive circuit 40 receives a nominal reference DC voltage signal, referred to as BIAS, and is coupled to and monitors the 12 V signal. The error amplifier and drive circuit 40 provides an output signal to the anode of the diode 26. A load resistor 42 is also provided between the 12 V signal and the 12 V RTN signal, which usually has a higher resistance than the resistor 34 and thus consumes less power than the resistor 34. The magnetic amplifier regulator 36 method provides good regulation using "feedback" control through operation of the error amplifier and drive circuit 40 to maintain the output voltage within a specified voltage range under most load conditions. This method is very costly, however, due to the added magnetic inductor 38, and also suffers from a very high parts count due to the member of parts required to implement the error amplifier and drive circuit 40. Circuitry must also be provided to generate the BIAS reference signal.
FIG. 1C illustrates another prior art method called a buck switch regulator 44. In the buck switch regulator 44, an error amplifier and drive switch rectifier 46 is coupled to the 12 V RTN signal and to the junction between the inductor 30 and the capacitor 32, which provides an input signal. The error amplifier and drive switch rectifier 46 receives and monitors the 12 V signal as feedback and provides an output signal to an added LC filter stage. The LC filter stage comprises a capacitor 50 coupled between the 12 V signal and the 12 V RTN signal, and an inductor iron 48 which receives the output signal of the error amplifier and drive switch rectifier 46 at one terminal and provides the 12 V signal at its other terminal. The buck switch regulator 44 also regulates the output voltage well, but is more sophisticated than the magnetic amplifier regulator 36 of FIG. 1B, and thus is even more costly, having an even greater parts count.
Finally, FIG. 1D illustrates another 12 volt supply 52 according to yet another prior art technique, which uses a 3-terminal regulator 54. The 3-terminal regulator has its input connected to the inductor 30, its output providing the 12 V signal and its adjust terminal coupled to the 12 V RTN signal. Another capacitor 56 is coupled between the 12 V signal and the 12 V RTN signal. The 3-terminal regulator 54 could be the LM317 manufactured by National Semiconductor, or could be another common type of 3-terminal regulator. This technique also regulates well but suffers from relatively low efficiency. Its cost is not excessive but is still significant.
It is therefore desirable to provide a low cost, highly efficient regulation circuit to regulate one of the relatively low regulated outputs of a forward converter switching power supply, such as the 12 volt output, without a significant number of additional parts.